Design Considerations of Forgings | Machine Design

In this post, we are going to discuss design considerations of Forgings. Forged components are widely used in the automotive and aircraft industries. They are usually made of steel and nonferrous metals. They can be as small as a gudgeon pin and as large as a crankshaft.
Forging is an engineering manufacturing process, in which solid metal is pressed under pressure usually by hammer to undergo extensive plastic deformation into finished or near to finish products. Forging produces fiber structure and brings metallurgical changes in the forged material. Forging is generally carried out on a hot workpiece.

Forged components are used under the following circumstances

  1. Moving components requiring lightweight to reduce inertia forces. Eg. connecting rod of IC engine.
  2. Components that are subjected to excessive stresses. Eg. Aircraft structure.
  3. Small components that must be supported by other structures or parts. Eg. Hand tools and handle.
  4. Components requiring pressure tightness where the part must be free from internal cracks. Eg. valve bodies.
  5. Components whose failure would cause injury and expensive damage are forged for safety.

Design Considerations of Forging

  • Keep fiber lines parallel to tensile forces and perpendicular to shear forces.
  • Provide an adequate draft angle.
  • Keep the parting line in one plane  as far as possible
  • Provide adequate fillet and corner radii
  • Avoid ribs and thin sections

In order to obtain maximum benefit from forged components, the following principles should be adopted:

Keep fiber lines parallel to tensile forces and perpendicular to shear forces

While designing a forging, an advantage should be taken of the direction of fiber lines. The grain structure of a crankshaft manufactured by the three principle methods, i.e. casting, machining, and forging are shown in figure 1.

Grain structure, design consideration of forging, casting
Fig. 1 Grain Structure.

There are no fiber lines in the cast component and the grains are scattered. In the case of a component, prepared by machining methods, such as turning or milling, the original fiber lines of rolled stock are broken.

It is only in the case of forged parts that the fiber lines are arranged in a favorable way to withstand stresses due to external load.

While designing a forging, the profile is selected in such a way that fiber lines are parallel to tensile forces and perpendicular to the shear forces. Machining that cuts deep into the forging, should be avoided otherwise the fiber lines are broken and the part becomes weak.

Provide an adequate draft angle

The forged component should be provided with an adequate draft as shown in figure 2.

Draft for forging
Fig. 2 Draft for forging

The draft angle is provided for easy removal of the part from the die impressions. The angles α and β are drafts for outside and inside surfaces.

As the material cools, it shrinks and a gap is formed between the outer surface of forging and the inner surface of the die cavity, with the result that the draft angle for the outer surface is small.

On the other hand, when the material cools, its inner surface tends to shrink and grip the projecting surface of the die, with the result that the draft angle for the inner surface is large.

For steels, the recommended values of α and β are 7° and 10° respectively.

Keep the parting line in one plane as far as possible

There are two important terms related to forgings as shown in figure 3.

Design considerations of forging

A parting line is a plane in which the two halves of the forging dies meet and in which flash is formed. A forging plane is a plane, which is perpendicular to the die motion. In most of the cases, the parting line and forging plane coincide, as shown in figure 4.

Location of parting line and forging plane
Fig. 4 Location of parting line and forging plane.

There are two basic principles for the location of the parting line – the parting line should be in one plane as far as possible and it should divide the forging into two equal parts.

When the parting line is broken as shown in figure 5 it results in unbalanced forging forces, which tend to displace the two die halves. Such forces are balanced either by a counter lock or by forging the two components simultaneously in a mirror image position.

A parting line that divides the forging into two halves ensures the minimum depth to which the steel must flow to fill the die impressions.

Unbalanced forces
Fig. 5 Unbalanced forces

Provide adequate fillet and corner radii

The forging should be provided with adequate fillet and corner radii. A small radius results in folds on the inner surface and cracks on the outer surface.

A large radius is undesirable, particularly if the forged component is to be machined, during which the fiber lines are broken.

Sharp corners result in increasing difficulties in filling the material, excessive forging forces, and poor die life. The magnitude of the fillet radius depends upon the material, the size of forging, and the depth of the die cavity.

For moderate size steel forgings, the minimum corner radii are 1.5, 2.5, and 3.5 mm for the depths up to 10, 25, and 50 mm respectively.

Avoid ribs and thin sections

Thin sections and ribs should be avoided in forged components. A thin section cools at a faster rate in the die cavity and requires excessive force for plastic deformation.

It reduces the die life and the removal of the component from the die cavities becomes difficult. For steel forgings, the recommended value of minimum section thickness is 3 mm.

A properly designed forging is not only sound with regard to strength but it also helps to reduce the forging forces, improves die life, and simplifies the design. If the design is poor, the best steel and forging methods will not give a satisfactory component.

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